At any boss battle you fight (don't try this tip on small battles like J.J. and the Bella Sisters) bring a rocket launcher. I was reading other entries and can't believe that people haven't found out about it. All bosses can be brought down by one shot from the rocket launcher.
At the El Gigante part, just shoot it once with the rocket. You don't have to do that stupid run up and cut. One shot and it dies.
At the Lake, just fight it out using those spears. They really should have thought that part out more at Capcom.
When fighting Cheif Mendaz, shoot him with the shotgun until his legs fall off. Then shoot him with the rocket. At the end, his legs will still be walking when he is dead if you can do it fast enough.
When fighting Salazar's right hand, wait for him to come down and fight, and knock over the nearest tank when he is close and shoot him once with the rocket.
When fighting Salazar, I suggest to shoot his eye with a magnum and when he is open, shoot him with the rocket. Watch out though, because he will still attack while you are aiming.
When fighting It, just keep moving at the part with the huge cages. When you get to the part with the cave, just shoot him with the rocket. If you have a lot of ammo, though, use the shotgun to save a couple bucks.
Now, you fight Krauser. Krauser is difficult. At the first part, I suggest moving up the ladder in the small room and shoot him with the shotgun. When he disappears, open the door. At this next part, keep running and dodgind his shots. When you get the the symbol thing, get under cover until he comes down. When he does that, start blasting him, move the statue and run to the now opened door. Go through that part and don't respond. Go up the stairs and all that stuff, and when he attacks again, dodge him and try to get him down on his knees, a bit away from you. Shoot him with the rocket launcher and then start shooting him with a magnum or a shotgun if he survives, which he often does. Run and open the door.
When you get to Saddlar, don't shoot him immediatly like in the other hints. No offense. Hit one of his eyes on his legs and when he gets down, shoot him in the eye with the rocket, so you don't have to get close and knife him several times. Then shoot him with the special rocket.
When you win the game, I suggest saving up for the Infinite Rocket Launcher. Once you get that, you can blast the smaller bosses and when you shoot people, the explode. It never gets entertaining. And instead of using the special rocket launcher in the final battle, you can just keep shooting him with the IRL and sell the special rocket launcher. It isn't worth much compared to other weapons, so i don't advise it if you have to clear out room.